COOPERATIVE BINDING OF DROSOPHILA HEAT-SHOCK FACTOR TO ARRAYS OF A CONSERVED 5 BP UNIT

Drosophila heat shock factor (HSF) exists as a multimer in solution and when bound to its regulatory element (HSE). We have previously reported evidence that subunits of HSF associate to form homotrimers and that each subunit contacts a conserved 5 bp DNA sequence repeated within an HSE. Here we show that HSF binding is highly cooperative at two distinct levels: between subunits of the HSF multimer, and between multimers. The binding of HSF to one of a pair of adjacent trimeric binding sites facilitates HSF binding to the second by over 2000-fold. This cooperativity is particularly important in binding HSF at 37-degrees-C, and could account for the requirement for multiple binding sites in vivo and, in part, for the differential expression of heat shock genes.